The present invention relates to dispersion compensation fibers for compensation of dispersion in glass fiber
Dispersion comensation fibers are described, for example, by D. M. Pataca, M. L. Rocha, K. Smith, T. J. Whitley, R. Wyatt in xe2x80x9cActively modelocked Pr3+-doped fluoride fibre laserxe2x80x9d Electr. Lett., 30 (1994) 2, p. 964.
The use and design of such dispersion compensation fibers (DC fibers) for compensation of the dispersion of the active fibers of the fiber laser with a fiber laser structure such as that presented in FIG. 1, for example, is already known. The phase modulator in the laser resonator requires both at the input and at the output defined linear polarized light which must be produced. Previously, this has been achieved using polarization convertors, which is generally complex and laborious.
As the radiation source of ultra-high bit rate transmission systems and as a source of solitons, modelocked fiber lasers are used to advantage. The most important prerequisite is that the pulse width over time must be as small as possible, i.e., it must not exceed a few ps. Various authors, such as D. M. Pataca, M. L. Rocha, K. Smith, T. J. Whitley, R. Wyatt: xe2x80x9cActively modelocked Pr3+-doped fluoride fibre laserxe2x80x9d Electr. Lett., 30 (1994) 2, p. 964, have demonstrated that the chromatic dispersion of the active fibers of the fiber laser has a pulse widening effect. The formulas by D. J. Kuizenga, A. E. Siegman: xe2x80x9cFM and AM Mode Locking of the Homogeneous Laserxe2x80x94Part I: Theory.xe2x80x9d IEEE J. Quant. Electr. 6 (1970), p. 694 with the supplement by G. Geister: xe2x80x9cIntegrierte optische Modulation von Nd-Faserlasemxe2x80x9d [Integrated optical modulation of Nd fiber lasers] Fortschrittsberichte VDI Reihe [VDI Progress Reports Series] 10 (1990) 140, 1, 102 describe the dependence of pulse half-width xcfx84p over time on modulation frequency fm, modulation index xcex4c, laser wavelength xcex, length of active fiber La, gain coefficient g and spectral half-width xcex94xcex of the fluorescence spectrum:       τ    p    =                                                                        2                ⁢                                  2                                ⁢                ln                ⁢                                  xe2x80x83                                ⁢                2                                      π                    ⁡                      [                          1                                                f                  m                  2                                ⁢                                  δ                  c                                                      ]                                    1          /          4                    ⁡              [                                            (                                                                                          λ                      2                                        ⁢                                          L                      a                                                                            2                    ⁢                    π                    ⁢                                          xe2x80x83                                        ⁢                    c                                                  ⁢                D                            )                        2                    +                                    (                              g                                                      π                    2                                    ⁢                  Δ                  ⁢                                      xe2x80x83                                    ⁢                                      f                    2                                                              )                        2                          ]                    1      /      8      
The extension by Geister is expressed by the additional term with D, taking into account chromatic dispersion.
FIG. 2 shows the negative influence of dispersion D on pulse half-width for the case of a Pr3+ ZBLAN glass fiber laser. This means that D must disappear in order to minimize xcfx84p, i.e., dispersion must be compensated. This can be accomplished by using a chirped fiber Bragg grating as the laser reflector, e.g., as the decoupling reflector. However, this method is very problematical. The reflecting power of the chirped fiber grating is precisely defined by optimizing the fiber laser and must thus be verified because otherwise the laser threshold is increased and the output power is reduced. To compensate for the dispersion of the active fibers the spectral half-width must be  greater than 10 nm according to estimates; otherwise xcfx84p is increased. These technological requirements cannot be met at the present time.
An object of the present invention is to permit a combination of the effects of dispersion compensation and definition of the linear polarization state in a glass fiber that has high birefringence of the DC fibers and is linked to the active fiber.
The present invention provides a dispersion compensation fiber for compensation of dispersion in a glass fiber, which is accommodated in the fiber laser resonator and linked to the active fiber, characterized in that it is formed by a selected doping in the preform and by a controlled elliptical shaping of the core for geometric birefringence which allows only the propagation of the two orthogonal linear polarization states, both for dispersion compensation as well as for definition of the linear polarization state.